Pension plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retirement Benefits [Abstract]
Service cost	$ 638	$ 718	$ 1,900	$ 1,624
Interest cost	62	66	183	149
Expected return on plan assets	(177)	(244)	(526)	(552)
Amortization of:
Prior service (benefit)	(5)	(6)	(14)	(14)
Net actuarial loss	49	0	146	0
Participant contributions	(196)	(217)	(584)	(491)
Net periodic benefit cost	$ 371	$ 317	$ 1,105	$ 716